1. Field of the Invention
The present invention relates to an improved high-speed, multi-break electrical switch. More specifically, the present invention constitutes a specific improvement of the switch claimed in commonly-assigned U.S. patent application, Ser. No. 21,646, filed Mar. 19, 1979 in the name of Otto Meister, now U.S. Pat. No. 4,342,978, and in a commonly-assigned, co-filed U.S. patent application, Ser. No. 179,366, filed Aug. 18, 1980 in the name of Raymond P. O'Leary, now abandoned.
2. Brief Discussion of Prior Work
The switch claimed in the above-noted Ser. No. 21,646 patent application constitutes an improvement of the switch claimed in priorly filed, commonly-assigned U.S. patent application, Ser. No. 972,650, filed Dec. 21, 1978 in the name of Otto Meister, now abandoned. The Ser. No. 972,650 application discloses a high voltage device which includes a fuse and a switch. The fuse is preferably a current-limiting fuse having a low continuous current rating which is normally shunted by the switch which has a high continuous current rating. When a fault current or other over-current occurs in the circuit to which the device is connected, the switch opens, commutating the current to the current-limiting fuse for limitation and interruption thereof. In the Ser. No. 972,650 application, the switch includes a pair of normally engaged contacts which are rapidly movable apart by the ignition of the power cartridge contained within a chamber defined by at least one of the contacts. The contacts move apart in a fixed line of direction forming a gap therebetween upon ignition of the power cartridge to open the switch and to effect current commutation to the current-limiting fuse. In preferred embodiments of the Ser. No. 972,650 application, the fuse coaxially surrounds the switch and both are contained in a common housing. This structure minimizes manufacturing costs, renders the overall device convenient to handle and manipulate, and minimizes the inductance of the overall device to ensure rapid commutation of the current from the switch to the fuse.
After the invention of the Ser. No. 21,646 application was made, the desirability of ensuring the rapid movement apart of the contacts upon ignition of the power cartridge was recognized, as was the fact that the ignition products of the power cartridge could well contaminate the gap which is created between the contacts. Such contamination of the gap may sufficiently lower the voltage of any arc forming in the gap between the contacts, which in turn may slow up or prevent rapid commutation of current from the switch to the fuse. In recognition of these problems, the switch of the Ser. No. 21,646 application includes a piston mounted on one of the contacts which is movable away from the other contact which may be stationary. The piston defines a portion of the closed chamber which contains the power cartridge and is movable, as its contact moves, through an insulative sleeve against which it seals. The gap between the contacts is, accordingly, electrically insulated and any arc formed between the contacts is constricted between the piston and the sleeve. Moreover, the piston is so related to its contact and the sleeve that it ensures rapid movement of its contact away from the stationary contact and isolates its contact from the ignition products of the power cartridge. In preferred embodiments, the piston and the sleeve are both made of an ablative, arc-extinguishing material. Thus, arc-extinguishing gas is generated by the piston and the sleeve during arc constriction. The constriction and the gas raise the arc voltage resulting in rapid commutation of the current from the switch to the fuse.
The Ser. No. 21,646 application also contemplates that the contact mounting the piston is normally electrically connected to a terminal of the device. The terminal is connectable to one side of a circuit being protected, while the stationary contact is connectable to the other side of the circuit. The electrical connection between the movable contact and the terminal is claimed in that application as comprising a variety of sliding contacts.
The invention of the co-filed, commonly-assigned application in the name of O'Leary is in turn an improvement of the invention of the Ser. No. 21,646 application. Specifically, the O'Leary application contemplates the formation of a second electrically insulated gap in addition to the electrically insulated gap opened between the contacts. More specifically, the O'Leary application contemplates the use of electrical connections between the movable contact and the terminal having certain sliding contact elements and insulative elements. Upon movement of the contact mounting the piston, the sliding contact elements and insulative elements co-act so as to form a second insulated gap. The two insulated gaps improve commutation of the current from the switch to the fuse.
The inventions of both the Ser. No. 21,646 and O'Leary applications both contemplate normally electrically interconnecting the switch contacts with a shearable or severable diaphragm-like member. Upon ignition of the power cartridge and movement apart of the switch contacts, the diaphragm is severed, torn, ripped or otherwise rendered discontinuous, thus breaking the normal electrical interconnection therebetween to open the switch. The use of a shearable diaphragm offers several advantages over sliding contact elements. First, in its normal state, the diaphragm is an integral, continuous member capable of carrying high currents. Sliding contact elements must be able to normally carry current across the interface therebetween when they are stationary and frictionally engaged, and to subsequently freely frictionally slide or move relative to each other. This latter property can result in a compromise in the first property. That is, because the normal frictional engagement between the elements must be such as to permit subsequent free relative movement of the elements, the current-carrying ability of the interface may be less than that of a solid or integral conductor. Second, when integral, the diaphragm positively prevents movement of the switch contacts between which it is connected. Sliding contact elements being only normally frictionally engaged cannot as positively prevent movement of the switch contacts they normally electrically interconnect. Third, substantial force is required to sever or tear the diaphragm. This permits the pressure of the ignited power cartridge to "build up" against the piston until the diaphragm is severed, thus ensuring rapid movement apart of the switch contacts. Sliding contact elements cannot ensure this rapid movement apart due, in great part, to the requirement, discussed above, that the elements be relatively movable.
Accordingly, the present invention specifically contemplates using such a shearable diaphragm or similar structure in the switch of the O'Leary application to both normally electrically interconnect the switch contacts and to normally electrically interconnect the one contact mounting the piston to the terminal. Both diaphragms are severed or otherwise rendered discontinuous upon ignition of the power cartridge. More generally, the present invention contemplates a switch capable of opening more than the two gaps opened by the switches of the Ser. No. 21,646 and O'Leary applications. Each such gap is opened by severing a diaphragm or similar structure.
The present invention also contemplates a "building block" approach to the construction of electrical switches. More specifically, the present invention recognizes that individual switch cells of two different types having specific characteristics may be alternated in a side-by-side series to produce a switch in which a multiplicity of insulated gaps are simultaneously opened upon ignition of a power cartridge. The fact that multiple insulated gaps are opened by the improved switch of the present invention means that after a very small amount of contact movement, numerous gaps--possibly with arcing therein--are formed to ensure rapid commutation of the current from the switch to the fuse.